Classical swine fever (CSF) or hog cholera is a highly contagious disease of pigs and wild boars. It causes fever, skin lesions, convulsions and usually (particularly in young animals) death within 15 days. The symptoms are indistinguishable from those of African swine fever. The disease is endemic in much of Asia, Central and South America, and parts of Europe and Africa. It was believed to have been eradicated in the United Kingdom by 1966 (according to the Department for Environment, Food & Rural Affairs), but an outbreak occurred in East Anglia in 2000. It was eradicated in the USA by 1978, according to the United States Department of Agriculture. Other regions believed to be free of CSF include Australia, Canada, Ireland, New Zealand and Scandinavia. While hog cholera does not cause foodborne illness in people, economic losses to pork producers would be severe if the disease were to become established again in any countries.
The most common method of transmission is direct contact between healthy swine and those infected with hog cholera. The disease can also be transmitted through contact with body secretions and excrement from infected animals. Healthy pigs coming into contact with contaminated vehicles, pens, feed, or clothing may contract the disease. Birds, flies, and humans can physically carry the virus from infected to healthy swine. Swine owners can inadvertently cause infection through feeding their herds untreated food wastes containing infected pork scraps.
The clinical signs of hog cholera vary with the severity of the infection. There are three forms of the disease: acute, chronic, and mild.
The acute form of hog cholera is highly virulent, causing persistent fevers that can raise body temperatures as high as 107° F. Other signs of the acute form include convulsions and lack of appetite. Affected pigs will pile or huddle up together. Signs of hog cholera may not be apparent for several days following infection. Death usually occurs within 5 to 14 days following the onset of illness.
The chronic form of hog cholera causes similar clinical signs in affected swine, but the signs are less severe than in the acute form. Discoloration of the abdominal skin and red splotches around the ears and extremities often occur. Pigs with chronic hog cholera can live for more than 100 days after the onset of infection.
The mild or clinically unapparent form of hog cholera seldom results in noticeable clinical signs. Affected pigs suffer short periods of illness often followed by periods of recovery. Eventually, a terminal relapse occurs. The mild strain may cause small litter size, stillbirths, and other reproductive failures. High mortality during weaning may also indicate the presence of this mild strain of hog cholera.
Hog cholera was the most devastating disease of swine. Containment and eventual eradication required research-based information on the cause and transmission of the disease as well as methods for diagnosis and prevention.
The infectious agent is a virus, CSFV (previously called hog cholera virus) of the genus Pestivirus in the family Flaviviridae (or Togaviridae). CSFV is closely related to the ruminant pestiviruses which cause Bovine Viral Diarrhoea (BVDV) and Border Disease (BDV) mainly affecting cattle.
The effect of different CSFV strains varies widely, leading to a wide range of symptoms. Highly virulent strains correlate with acute, obvious disease and high mortality, including neurological symptoms and hemorrhages within the skin. Less virulent strains can give rise to subacute or chronic infections that may escape detection, while still inducing mortality in fetuses and new-borns. Infected piglets birthed from infected but subclinical sows help maintain the disease within a population. Other symptoms can include lethargy, fever, immuno-suppression and secondary respiratory infections. The incubation period of CSFV ranges from 2 to 14 days but symptoms may be apparent after 2 to 4 weeks. Animals with an acute infection can survive 2 to 3 months before their eventual death.
Eradicating CSF is problematic. Current programs revolve around rapid detection and diagnosis, and preventive culling, possibly followed by emergency vaccination. Possible sources for maintaining and introducing infection include the wide transport of pigs and pork products, as well as endemic CSF within wild boar and feral pig populations.
Vaccination of pigs with a live attenuated classical swine fever virus (CSFV) vaccine strain, particularly the “Chinese” strain (LPC, also called C-strain), protects pigs against CSF. A major drawback of vaccinating pigs with the conventional LPC vaccines, is that the vaccinated pigs cannot be distinguished serologically from pigs infected with a CSFV field strain. The LPC, however, is considered one of the most effective and safe live vaccines. Discrimination of the vaccine-strain and the infectious virulent strain would be a great improvement
The genomes of pestiviruses consist of a positive strand RNA molecule of about 12.5 kb. The positive strand RNA genomes of several non-cytopathogenic BVDV strains, however, may be considerably larger.
Although less commonly available, nucleic acid testing (e.g., viral RNA or proviral DNA amplification method) can also help diagnosis in certain situations and has become more acceptable. The importance of nucleic acid testing (NAT) has become increasingly evident during the last decade for many purposes, such as diagnosing viral infections, monitoring antiviral therapy, and improving the safety of blood supplies. NAT combines the advantages of direct and highly sequence-specific detection of the genome of an infectious agent with an analytic sensitivity that is several orders of magnitude greater than that of antigen detection or virus isolation methods. NAT also reduces the risks of viral transmission during the period between infection and seroconversion, of infection with immunovariant viruses, of immunosilent carriage, and of occult carriage.
Single Nucleotide Polymorphism (SNP) is the most abundant form of genetic variation. Due to their frequency and distribution, SNPs are becoming superior genetic markers for identifying different species or virus strains. Recently, a number of methods for SNP detection have been developed, including restriction fragment length polymorphism (RFLP) analysis, single-strand conformation polymorphism analysis (SSCP), allele-specific oligonucleotide hybridization (ASO), oligonucleotide ligation as say(OLA), primer extension assay, and structure-specific flp nuclease technology. Another known method for SNP genotyping is allele-specific primer extension (ASPE). ASPE entails extension of a novel two component primer on templates which may or may not include a target nucleic acid sequence. The 3′ portion of the primer is complementary to a portion of the template adjacent the target sequence. The 5′ portion of the primer is complementary to a different preselected nucleic acid sequence. Extension of the 3′ portion of the primer with labeled deoxynucleoside triphosphates yields a labeled extension product if, but only if, the template includes the target sequence. The presence of such a labeled primer extension product is detected by hybridization of the 5′ portion to the preselected sequence as refer to Ye et al, Human mutation.17, p 305 (2001) and EP1302547A2, both of which are herein incorporated by reference in their entireties.
However, in the ASPE, it needs dCTP conjugated with biotin and extra enzymes to accomplish the reaction, wherein one is shrimp alkaline phosphotase, and the other is exonuclease I. Further, the major drawback of ASPE is that the reaction is amplified in single strand one strand at a time, in which DNA polymerase copies the DNA template, starting at the primer annealed to one of its strand, not both. Therefore, the amplification of DNA template is an arithmetic progression.
All referenced patents, applications and literatures are incorporated herein by reference in their entirety. Furthermore, where a definition or use of a term in a reference, which is incorporated by reference herein is inconsistent or contrary to the definition of that term provided herein, the definition of that term provided herein applies and the definition of that term in the reference does not apply. The invention may seek to satisfy one or more of the above-mentioned desire. Although the present invention may obviate one or more of the above-mentioned desires, it should be understood that some aspects of the invention might not necessarily obviate them.